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29 Cards in this Set
- Front
- Back
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First stage of transcription?
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Initiation -- sequence 'motifs' in the promoter are recognized by
transcription factors (TFs). TFs know where to position themselves based on the TATA box, which is always a set distance away from where the polymerase needs to start from. |
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What else do the TFs do?
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Bind the DNA and RNA polymerase, and direct the latter to the transcription initiation point.
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How do TFs know where to go?
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(1) factors recognize nucleotide motifs by binding
nitrogenous bases in the major & minor grooves of DNA |
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Coding strand and template strand? Which one is used?
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Coding is on top (5' to 3'), and template is on the bottom (3' to 5'). RNA polymerase complements the template strand, and is antiparrallel.
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Does transcription require a primer?
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NO!
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Eukaryotes use different RNA Pol's for different transcript types
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RNA Polymerase I --> rRNA
RNA Polymerase II*** --> mRNA *** RNA Polymerase III --> tRNA (and a rRNA) |
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Second stage?
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Elongation -- the RNA polymerase moves down the strand, kicking out histones, and transcribing as it goes.
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Third stage? How does this happen?
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Termination! Due to a termination sequence (AAUAAA) being generated in the pre-mRNA. 10-35 nts after the message, the RNA is cut off. It then has a 30-200 nt long adenine tail added (i.e., AAAAAAAAAAAAAAAAA), called a PolyA tail. These are not coded for, they are non-templated additions.
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What is the Poly(A) signal?
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AAUAAA, the termination sequence! **remember, it's AAUAAA in the mRNA, and TTATTT in the TEMPLATE STRAND.
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What is the 1st thing done post-transcribing?
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Methy-guanine cap is added to the 5' end phosphate.
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What is the methyl-guanine cap good for?
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-- protects the mRNA from enzymatic degradation by letting it know what is its, and what is not.
-- will eventually provide a recognition site for ribosomes |
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2nd thing done post?
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- removing 'introns', which are regions of non-coding RNA
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Why are introns there if they're not used?
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Allows, evolutionaryly, for random chance shuffling of genetic material that can lead to (in some cases) certain evolutionary advantages.
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How to exons work?
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They usually encode distinct functional domains of a polypeptide.
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How are the introns cut out of the premRNA?
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Spliceosome complex, a protein-RNA Complex that excises the introns.
Works by having the snRNA complement the premRNA's nucleodtides, and then indicates where to cut. The junction between the exon and intron has a distinct nucleotide segment that is recognized and bound by the snRNA and ribeonuclear complex, which directs the cutting. |
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Single stranded RNA lacks the double helix.. how does it read it then?
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Compensates with complementary nucleotides.
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3rd thing done post?
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Poly(A) tail is added, and the mRNA can now leave the nucleus.
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Anything past the stop codon will...
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... not be translated.
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How does mRNA control how much is used?
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It has a shelf-life of sorts, it eventually decays and falls apart.
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Coding segments of the mRNA? UTR segments?
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Coding segments encodes the polypeptide. The UTR (untranslated regions) are not encoded, and this helps regulate translation rates (?)
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Transcription in prok vs. Euka?
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In Euka, you have the intermediate steps of having mRNA
Pol II -- encodes pre-mRNA Pol I and III -- RNA and end procuts (e.g., snRNA). In prok, since there is less genetic information to worry about, they go straight to premRNA. This allows them to divide a lot faster. |
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Bacteria and Archaea transcription specifically?
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Bacteria -- a single Pol and simple promotors. Transcribes all RNA types.
Archea -- intermediate type of polymerase, but not nearly as complex. Archean RNA polymerase uses eukar-like promoter and TFs. Transcribes mRNA and end products. |
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What is the bacteria's unique initiation system for transcription?
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– Sigma factor (∂). 1st bind RNA pol and together these can position the RNA pol at its starting place on template strand.
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Other similarities/differences in elongation and termination in prok?
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Elongation -- - after initiation & onset of elongation the sigma factor falls off Pol
- transcription bubble & elongation similar to eukaryotes Termination -- - distinct termination sequences in prokaryotes. -- transcription stops right at termination signal in 3’ UTR - unlike eukaryotes - NO poly(A) tail is added |
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polycistronic
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Refers to the fact that one mRNA transcript can encode multiple different polypeptides in bacterial mRNA.
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Some other unique feature of prok transcription?
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Initial RNA transcript is mRNA
- no pre-mRNA or post-transcriptional modifications** - no 5' mG cap -no poly(A) tail - no introns - therefore bacterial mRNA is rapidly degraded - in minutes - 5' UTR & 3' UTR do exist |
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Main difference between prok and eukar transcription?
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Proak have mRNA ready to be translated, whereas Eukar have premRNA turned into mRNA, then ready for translation.
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What does the promoter and enhancers do in eukar transcription?
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Promoter -- defines transcription start site for RNA Polymerase (Pol)
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Transcription repressors?
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can block TF binding.
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